The reference is to ‘On dimorphism in the hymenopterous genus Cynips’, by Benjamin Dann Walsh (Walsh 1864a). CD’s annotated copy is in the Darwin Pamphlet Collection–CUL. See letter from B. D. Walsh, 29 April – 19 May 1864 and n. 7. Lubbock reviewed the paper in Natural History Review (Lubbock 1865a).